Solenoid valves are used for adjusting hydraulic pressures supplied to hydraulic devices, such as hydraulic pistons, actuators, clutches and brakes, which are operated by the hydraulic pressures, to be instructional pressures. Each of the solenoid valves is supplied with a current corresponding to the instructional pressure, so as to adjust an original pressure supplied from an oil pump or the like, and to supply the hydraulic pressure that is equal to the instructional pressure to the hydraulic device.
However, some solenoid valves have a dead band region, in which an actual pressure is hard to be reduced even though the instructional pressure is reduced, on a low pressure side. When the solenoid valve is controlled in that region, a change in the hydraulic pressure that is supplied to the device operated by the hydraulic pressure is delayed, and thus deterioration in control responsiveness is caused.
For this reason, according to JP2009-14138A, a change in the actual pressure, at the time when the instructional pressure is changed, is examined so as to identify the dead band region of the solenoid valve, and not to use the dead band region.